Range ventilating system

ABSTRACT

A venting system for a high console range incorporating a pair of propeller fans for operation in a restricted hollow riser between the range cooktop and the high console hood. The fans may be used interchangeably in either a room air recirculating system or a vented to the outside atmosphere system by virtue of the fan blades being located in side-by-side orifices in a vertical shroud wall forming a fan blade plenum chamber providing fan blade housing means in conjunction with either a removable air guide directing the air to a room vent operative with the recirculating system or a removable double-lobed shroud enclosing the fan blades for exhausting air by ducting through a building wall to the atmosphere. The adaptable housing thereby alters the fans&#39;&#39; axial characteristics to provide mixed radial and axial air flow.

United States Patent 1 1 Gould 1 1 March 6, 1973 1 RANGE VENTILATING SYSTEM Ira L. Gould, Hamilton, Ohio [73] Assignee: General Motors Corporation,

Detroit, Mich.

22 Filed: Dec. 1,1971

21 Appl.N0.: 203,737

[75] Inventor:

52 US. Cl. ..98/1l5 K, 126/229 R 51 1m. 01. ..F23j 11/00 581 Field of Search ..9s/11s K, 115 1.11, 115 VM,

115 R; 126/299 B, 299 R [56] References Cited UNITED STATES PATENTS 3,538,839 11/1970 Stalker ..98/1l5 3,109,358 11/1963 Meyer ..98/1 15 K Primary ExaminerWilliam F. ODea Assistant ExaminerPaul Devinsky Attorney-William S. Pettigrew et al.

[ 5 7] ABSTRACT A venting system for a high console range incorporating a pair of propeller fans for operation in a restricted hollow riser between the range cooktop and the high console hood. The fans may be used interchangeably in either a room air recirculating system or a vented to the outside atmosphere system by virtue of the fan blades being located in side-by-side orifices in a vertical shroud wall forming a fan blade plenum chamber providing fan blade housing means in conjunction with either a removable air guide directing the air to a room vent operative with the recirculating system or a removable double-lobed shroud enclosing the fan blades for exhausting air by ducting through a building wall to the atmosphere. The adaptable housing thereby alters the fans axial characteristics to provide mixed radial and axial air flow.

6 Claims, 7 Drawing Figures PATENTEU 6 I 73 SHEET 10F 2 I NVEN '1 Y IR. X122 11 Con/a W%M ATTORNEY RANGE VENTILATING SYSTEM This invention relates to range exhaust systems and more particularly to a high console domestic range venting system which may be readily adapted to either a room recirculating system or an outside vented system by means of a pair of inexpensive constant speed propeller fans.

While propeller fans have been proposed for use in ventilated high console domestic ranges the prior art, exemplified by U.S. Pat. No. 3,538,839 issued Nov. 10, 1970 to J. E. Stalker, does not teach an arrangement providing a pair of propeller fans for installation in a plenum chamber of a high console range for pressurizing the plenum chamber which may be readily altered to either a room air recirculating venting system or to exhausting cooking vapors to the atmosphere by ducting through a building wall.

It is an object of this invention to provide an efficient, inexpensive venting arrangement for high console domestic range having an inverted L-shaped hollow riser and hood backguard assembly employing a pair of inexpensive constant speed propeller fans whose blades are surrounded by side-by-side cylindrical orifices in a vertical plenum chamber located in the riser such that the fans may be used interchangeably in either a room air recirculation system or in a vented to the outside system by means of removable air directors configured to convert the axial flow fans into mixed radial and axial flow impellers.

Further objects and advantages of the present invention will be apparent from the following description, reference being had to the accompanying drawings wherein a preferred embodiment of the present invention is clearly shown.

In the drawings:

FIG. 1 is a fragmentary perspective view of a high console range embodying the present invention.

FIG. 2 is a partially sectioned side view of the backguard assembly of the range of FIG. 1.

FIG. 3 is a vertical section of the backguard assembly taken on the line 33 of FIG. 2 with portions of the fan housing broken away.

FIG. 4 is an exploded view of the riser dividing wall and removable housing.

FIG. 5 is a vertical section view similar to FIG. 3 with the range adapted for its recirculating mode.

FIG. 6 is a partially sectioned fragmentary side view of the range backguard of FIG. 5.

FIG. 7 is an exploded view of the duct dividing wall structure of FIGS. 5 and 6.

Referring now to the drawings and particularly to FIGS. 1 and 2, a free standing high console kitchen range unit is indicated generally by the numeral 10. The unit consists of a lower range section 11 including surface cooking element 12 located above a cabinet including an oven (not shown). While the surface elements and oven are preferably heated by electric resistance heaters, other means such as gas may be used for the heating mode without departing from the scope of the disclosed invention. The surface elements may be controlled by conventional switch means (not shown).

The range unit includes a backguard assembly 13 enclosure of generally inverted L-shape having a vertical hollow riser portion or midsection 14, defined in part by forward splash panel 15 and back panel 16, and

an upper hood portion 17 defined by a metal top deck 18 which could be replaced by a suitable glass warming shelf, if desired, and a slightly outwardly and upwardly inclined bottom panel 19 and a hood front panel 20. It will be noted that the vertical hollow riser portion 14 has a relatively narrow depth dimension A (FIG. 1) relative to its height and width. Further the height B (FIG. 2) of the hood portion 17 is small relative to its width and depth and the depth C (FIG. 1) of the hood 17 is limited with the result that a satisfactory maximum or high blower air flow of the order of 300 cubic feet per minute cannot be obtained with a conventional blower fan while being accommodated in the restricted hood space. In the preferred form the depth dimension A is about 3 inches, the height dimension B is about 5 inches and the hood depth dimension C is about 13 inches. The hoodportion 17 includes a convenience light 22, reflector 23 and glass illumination plate 24 which extend transversely the full width of the hood.

The range splash panel 15 has a smoke and grease filter 26 of glass or metal fibers which could include charcoal to filter out odors. The filter 26 is removably supported on upper and lower guide rails 34 and 36 for sliding removal.

Within the hood portion 17 is an air translation system including fan means in the form of a pair of propeller fans 40 and 42 (FIG. 3) including drive motors, one of which is shown at 44 in FIG. 2, each having substantially horizontal drive shafts 45 and 46 extending toward and substantially perpendicular to the rear wall 16. The fan propellers 47 and 48, having a high degree of blade pitch, are concentrically located within a pair of exhaust air flow cylindrical duct orifices 49 and 50, respectively, formed in a generally rectangular shroud wall member 51. The intake edges 52 of the propeller fan blades 47, 48 are in substantial alignment with the forward face of the shroud wall 51 with the duct orifices partially enclosing the fan blades. The shroud wall 51 extends vertically to the hood top deck 18 so as to define a high pressure plenum chamber 54 on the discharge side which extends vertically in the riser portion 14 and the hood 17 after receiving air from low pressure suction plenum or motor chamber 56.

The shroud wall 51 has inwardly directed upper and lower peripheral right angular flanges 58 and 60 respectively allowing the shroud wall 51 to be positioned in a vertical plane intermediate the front splash panel 15 and rear wall 16 whereby the upper flange 58 is secured as by spot welding in a sealed airtight manner to the underside of the top deck 18. The lower flange 60 is configured to provide a downwardly extending border 62 which is affixed in a sealed airtight manner to the flange splash panel 15. Lanced flange tabs 63 and 64 parallel to the plane of shroud 51 and offset from it are affixed to the splash panel 15 by suitable fasteners extending through holes 65. The shroud wall 51 is adapted to receive, interchangeably, either a housing or an air guide to be described. It is the interchangeability of the housing 70 with air guide 100 which makes the present construction adaptable to either an outside vented system or a recirculating system utilizing the pair of propeller fans 40 and 42 interchangeably.

As illustrated in FIGS. 1-4 the backguard assembly 13 includes the first removable fan shroud or impeller housing 70 preferably made of a plastic material such as bakelite formed in a one-piece or integral moulded manner to provide a U-shaped casing 71 and a scrollshaped casing 72 such that juxtaposed lobe segments 74 and 75 are connected along a cuspidal edge 73 at the intersection of the lobed segments 74 and 75 so as to terminate in a common tangential partition 76 formed integral with a rear baffle plate 77. The U- shaped casing 71 and the scroll-shaped casing 72 are arranged to feed a common restricted outlet 78 of generally rectangular shape formed in the baffle plate 77 for flush connection with rectangular rear wall air exit 126 (FIG. 6) and rectangular wall duct, partially indicated by dashed lines at 79 in FIG. 2, for venting to the outside. In the preferred form the common outlet 78 is dimensioned to mate in a flush manner with a rectangular opening in the rear wall dimensioned to receive a building wall duct 79 having a width of inches and a height of 3 inches which are the standard dimensions for a residential wall duct. Suitable sealing means such as polyurethane foam tape lining the edge of outlet 78 (not shown) are provided to seal the wall duct 79 within the outlet 78 and the rear wall opening 126.

The U-shaped casing 71 has its lobe segment 74 terminating in a downwardly directed side wall 82 parallel to the common partition 75 while the lobe segment 75 has a downwardly and inwardly inclined side wall 84. The side walls 82 and 84 define a lower concave wall portion 86 which curves rearwardly toward the exist 78. The concave wall portion 86 terminates at the horizontal upper edge 87 wherein upper openings 88 and 89 are provided for enclosing the cylindrical propeller fan orifices 49 and 50, respectively in an offset manner in their associated casings 71 and 72.

By virtue of the above-described structure the scrollshaped casing 72 and its propeller 48 provide a singleimpeller mixed-flow scroll unit in which the air flow is both radial and axial while the U-shaped casing 71 and its propeller 47 provide a single-impeller mixed flow U- shaped unit. It will be noted that to enable first and second fans 40 and 42 to exhaust to the common restricted riser rear wall exit 78 applicant has devised the composite double-lobed fan shroud housing 70 such that the casing 72 is in the form of a scroll to convert the high velocity pressure of propeller blades 48 to high static pressure at the scroll-shaped casing outlet located between partition wall 76 and the cut-off point 90. The cut-off point is defined as the closest point on the scroll to the center of the impeller and prevents to a certain degree the recirculation of air approaching the scroll casing outlet from the top of the impeller. The space between the scroll-casing 72 cutoff point 90 and the periphery of the blades 48 is defined as the blade exposure distance D, which in the instant form is of the order of 66 percent of the blade diameter. This is to be contrasted with the fan blade 47 of the U-shaped casing 71 which has a 100 percent blade exposure distance indicated at D, in FIG. 3. In the preferred form the distance D, is about 3.25 in. and D, is about 5.25 in.

By means of the composite housing design the propeller 48 and its scroll-casing 72 are capable of developing sufficient static pressure to enable the single propeller fan 42 to operate by itself for low speed exhaust while the operation of both propeller fans 40 and 42 provide for high speed exhaust. The fans 40 and 42 may be turned off and on by suitable switch means such as pushbutton switch 91. In this way two relatively inexpensive constant speed propeller fans may be used with the composite housing 70 to achieve an exhaust to outside air range venting system within a limited space providing both high and low exhaust speeds. In the preferred embodiment the fan propellers 47 and 48 have a diameter of 5.0 in., a blade depth of 0.97 in. and a maximum speed of about 3,450 revolutions per minute.

The housing member has arcuate outwardly directed flanges 92 and 93 and gusset flange 94 provided with suitably spaced holes 94 aligned with holes 96 in shroud wall 51 for the reception of fasteners 97 such as metal screws to removably secure the housing 70 in proper alignment with the cylindrical duct fan orifices 49 and 50 of the shroud wall.

Turning now to FIGS. 5, 6 and 7 it will be seen that the range backguard assembly 13 may be readily converted to 'a room air recirculating installation by simply interchanging the first removable means or shroud housing 70 and replacing it with a second removable means in the form of air guide 100. In the preferred form an air guide 100 is shown having a Z shape in cross section for location between the shroud wall 51 and the back panel 16. As seen in FIG. 5 the air guide 100 is generally channel shaped in plan and extends across the riser portion 14 below the shroud wall fan orifices 49 and 50 and above the rear wall air exit 126 to block same and upwardly on either side to the upper deck 18 to form a recirculating air plenum chamber 102.

With the room air recirculation system of FIGS. 5-7, the running of either or both fans 40 and 42 will draw smoke and greased laden fumes, indicated by the short arrows, through the filter 26 and upwardly through the duct portion 83 into the suction or low pressure plenum chamber 56 thence through the cylindrical duct orifices 49 and 50 and into the high pressure plenum chamber 102. The filtered air will then be forced upwardly through air vent means in the form of louvered grill in the upper end of the riser upper deck 18 and discharged back into the room.

The air guide 100 has outwardly directed flange 112 provided with spaced holes 114 for mating with aligned holes 116 on the shroud wall 51 for the reception of suitable fasteners 118. In a like manner the inwardly directed flange 120 has spaced holes 122 for alignment with holes in the back panel 16 for the reception of fasteners to effectively seal the recirculating air chamber 102. It will be noted that in the vented-to-outside atmosphere system the grill 110 is blocked off from orifice means 49 and 50 by the double-lobed shroud housing 70 while a plate 124 may be suitably attached to the back panel 16 to close the rear wall or back panel air exit 126, if desired. By virtue of the upwardly curved channel shaped form of air guide 100 the plenum chamber 102 serves to convert the axial flow fans into mixed fiow axial and radial fans for directing air upwardly through the room air vent means 1 10.

Thus, applicant has provided a structure wherein alternative duct exhaust to the outside or room recirculating systems are achieved without cutting, forming or other metal working operations during installation as required by systems presently on the market. It is merely necessary to secure either first or second removable fan housing means in the form of double-lobed shroud 70 or the channel-shaped air guide 100 in the high pressure plenum chamber in their appropriate positions by using suitable bolts or screws.

While the embodiment of the present invention as herein disclosed constitutes a preferred form, it is to be understood that other forms might be adopted.

lclaim:

l. A domestic cooking appliance comprising a range unit including a cooking top at waist height having front and rear surface heating units, a combination backguard and hood assembly of general inverted L- shape having a vertical hollow riser section adapted to rest upon the range unit at the rear edge of same, said riser section having a front vertical splash wall and a rear vertical wall spaced apart so as to provide a relatively narrow depth dimension relative to said riser sections height and width, said hood projecting forwardly from said riser section horizontally over generally the entire rear portion of the cooking top, the height of said hood section being small relative to the width and depth of same wherein the improvement comprises alternative range ventilation systems including an air inlet in the front splash wall of said risersection for admitting vapors rising from the cooking top thereto and an air exit in the rear wall of said riser section adapted for discharge by ducting through a building wall to the atmosphere, said splash wall air inlet and said rear wall air exit being substantially aligned, a vertical shroud wall spaced intermediate said front splash wall and said rear wall dividing said riser section into a front vertical low pressure air passage in communication with said splash wall air inlet and a rear vertical high pressure plenum chamber in communication with said rear wall air exit, room air vent means located at the top of said riser for discharging air from said plenum chamber to the room air, first and second fans having rotatable axial flow propeller blades supported in side-by-side relation in said hood, said shroud wall having a pair of propeller bladeorifice means arranged in side-by-side relation and surrounding respectively each of said first and second axial flow propeller blades, fan housing means including first and second removable means insertable in said plenum chamber for alternatively blocking said room air vent means or said rear wall air exit whereby to adapt said plenum chamber for recirculation of air from said splash wall air inlet to said room air vent means when said rear wall air exit is blocked and for exhausting air from said splash wall inlet to said rear wall air exit when said room air vent means is blocked, said first and second removable means each configured to convert said fan housing means into a mixed radial and axial flow fan, said first removable room air vent blocking means including a unitary double-lobed fan shroud having first and second apertured casings for enclosing respectively said first and second propeller blades, each casing having a separate downwardly opening outlet, said double-lobed shroud having a common exit means providing air passage communication between said casing outlets and said rear wall air exit, said second removable rear wall air exit blocking means including an air guide for positioning in said plenum chamber above said rear wall air exit and below said pair of propeller blade orifice means providing air passage communication between said orifice means and said room air vent means.

2. The combination of claim 1 wherein said first casing is U-shaped such that its outlet provides 7 unrestricted blade exposure relative to its casing outlet and said second casing is scroll-shaped such that its outlet provides about two-thirds blade exposure relative to its casing outlet to enable said second fan to develop sufficient static pressure such that said second fan may be operated singly to provide for normal air exhaust through a building wall ducting to the atmosphere and said first and second fans may be operated in unison to provide for high air exhaust flow to the atmosphere.

3. A domestic cooking appliance comprising a range unit including a cooking top at waist height having front and rear surface heating units, a combination backguard and hood assembly of general inverted L- shape having a vertical hollow riser section adapted to rest upon the range unit at the rear edge of same, said riser section having a front vertical splash wall and a rear vertical wall spaced apart so as to provide a relatively narrow depth dimension relative to said riser sections height and width, said hood projecting forwardly from said riser section horizontally over generally the entire rear portion of the cooking top, the height of said hood section being small relative to the width and depth of same wherein the improvement comprises an atmospheric air exhaust system including an air inlet in the front splash wall of said riser section for admitting vapors rising from the cooking top thereto and an air exit in the rear wall of said riser section adapted for discharge by ducting through a building wall to the atmosphere, said splash wall air inlet and said rear wall air exit being substantially in alignment, a vertical shroud wall spaced intermediate said front splash wall and said rear wall dividing said riser section into a front vertical low pressure air passage in communication with said splash wall air inlet and a rear vertical high pressure plenum chamber in communication with said rear wall air exit, first and second rotatable propeller single speed fans having rotatable propeller blades supported in side-by-side relation in said hood, said shroud wall having a pair of propeller blade orifice means arranged in side-by-side relation surrounding respectively each of said first and second axial flow propeller blades, a unitary double-lobed fan housing positioned in said plenum chamber, each lobe of said doublelobed fan housing having offset first and second apertures receiving said first and second orifice means therethrough forming a first U-shaped casing enclosing said first propeller blade and a second scroll-shaped casing enclosing said second propeller blade, each said casing having downwardly opening air outlets, said double-lobed housing having a common air exit positioned in aligned flush communication with said rear wall air exit, said U-shaped casing and said scrollshaped casing operative to convert said first and second axial flow propeller fans into mixed radial and axial flow fans, said U-shaped casing enclosing said first propeller blade in a manner whereby said first blade has unrestricted exposure to its casing outlet and said scroll-shaped casing enclosing said second blade, whereby said second blade has about two-thirds exposure to its casing outlet to enable said second fan to develop sufficient static pressure such that it may be energized singly to provide normal air exhaust flow through a building wall ducting to the atmosphere while said first fan may be energized to run in unison with said second fan to provide for high air exhaust flow to the atmosphere.

4. The claimed subject matter of claim 3 wherein said rear wall air exit has a restricted rectangular shape of a size to mate with a conventional rectangular wall duct having the dimensions of approximately 3 inches 

1. A domestic cooking appliance comprising a range unit including a cooking top at waist height having front and rear surface heating units, a combination backguard and hood assembly of general inverted L-shape having a vertical hollow riser section adapted to rest upon the range unit at the rear edge of same, said riser section having a front vertical splash wall and a rear vertical wall spaced apart so as to provide a relatively narrow depth dimension relative to said riser section''s height and width, said hood projecting forwardly from said riser section horizontally over generally the entire rear portion of the cooking top, the height oF said hood section being small relative to the width and depth of same wherein the improvement comprises alternative range ventilation systems including an air inlet in the front splash wall of said riser section for admitting vapors rising from the cooking top thereto and an air exit in the rear wall of said riser section adapted for discharge by ducting through a building wall to the atmosphere, said splash wall air inlet and said rear wall air exit being substantially aligned, a vertical shroud wall spaced intermediate said front splash wall and said rear wall dividing said riser section into a front vertical low pressure air passage in communication with said splash wall air inlet and a rear vertical high pressure plenum chamber in communication with said rear wall air exit, room air vent means located at the top of said riser for discharging air from said plenum chamber to the room air, first and second fans having rotatable axial flow propeller blades supported in side-by-side relation in said hood, said shroud wall having a pair of propeller blade orifice means arranged in side-by-side relation and surrounding respectively each of said first and second axial flow propeller blades, fan housing means including first and second removable means insertable in said plenum chamber for alternatively blocking said room air vent means or said rear wall air exit whereby to adapt said plenum chamber for recirculation of air from said splash wall air inlet to said room air vent means when said rear wall air exit is blocked and for exhausting air from said splash wall inlet to said rear wall air exit when said room air vent means is blocked, said first and second removable means each configured to convert said fan housing means into a mixed radial and axial flow fan, said first removable room air vent blocking means including a unitary double-lobed fan shroud having first and second apertured casings for enclosing respectively said first and second propeller blades, each casing having a separate downwardly opening outlet, said double-lobed shroud having a common exit means providing air passage communication between said casing outlets and said rear wall air exit, said second removable rear wall air exit blocking means including an air guide for positioning in said plenum chamber above said rear wall air exit and below said pair of propeller blade orifice means providing air passage communication between said orifice means and said room air vent means.
 1. A domestic cooking appliance comprising a range unit including a cooking top at waist height having front and rear surface heating units, a combination backguard and hood assembly of general inverted L-shape having a vertical hollow riser section adapted to rest upon the range unit at the rear edge of same, said riser section having a front vertical splash wall and a rear vertical wall spaced apart so as to provide a relatively narrow depth dimension relative to said riser section''s height and width, said hood projecting forwardly from said riser section horizontally over generally the entire rear portion of the cooking top, the height oF said hood section being small relative to the width and depth of same wherein the improvement comprises alternative range ventilation systems including an air inlet in the front splash wall of said riser section for admitting vapors rising from the cooking top thereto and an air exit in the rear wall of said riser section adapted for discharge by ducting through a building wall to the atmosphere, said splash wall air inlet and said rear wall air exit being substantially aligned, a vertical shroud wall spaced intermediate said front splash wall and said rear wall dividing said riser section into a front vertical low pressure air passage in communication with said splash wall air inlet and a rear vertical high pressure plenum chamber in communication with said rear wall air exit, room air vent means located at the top of said riser for discharging air from said plenum chamber to the room air, first and second fans having rotatable axial flow propeller blades supported in sideby-side relation in said hood, said shroud wall having a pair of propeller blade orifice means arranged in side-by-side relation and surrounding respectively each of said first and second axial flow propeller blades, fan housing means including first and second removable means insertable in said plenum chamber for alternatively blocking said room air vent means or said rear wall air exit whereby to adapt said plenum chamber for recirculation of air from said splash wall air inlet to said room air vent means when said rear wall air exit is blocked and for exhausting air from said splash wall inlet to said rear wall air exit when said room air vent means is blocked, said first and second removable means each configured to convert said fan housing means into a mixed radial and axial flow fan, said first removable room air vent blocking means including a unitary double-lobed fan shroud having first and second apertured casings for enclosing respectively said first and second propeller blades, each casing having a separate downwardly opening outlet, said double-lobed shroud having a common exit means providing air passage communication between said casing outlets and said rear wall air exit, said second removable rear wall air exit blocking means including an air guide for positioning in said plenum chamber above said rear wall air exit and below said pair of propeller blade orifice means providing air passage communication between said orifice means and said room air vent means.
 2. The combination of claim 1 wherein said first casing is U-shaped such that its outlet provides unrestricted blade exposure relative to its casing outlet and said second casing is scroll-shaped such that its outlet provides about two-thirds blade exposure relative to its casing outlet to enable said second fan to develop sufficient static pressure such that said second fan may be operated singly to provide for normal air exhaust through a building wall ducting to the atmosphere and said first and second fans may be operated in unison to provide for high air exhaust flow to the atmosphere.
 3. A domestic cooking appliance comprising a range unit including a cooking top at waist height having front and rear surface heating units, a combination backguard and hood assembly of general inverted L-shape having a vertical hollow riser section adapted to rest upon the range unit at the rear edge of same, said riser section having a front vertical splash wall and a rear vertical wall spaced apart so as to provide a relatively narrow depth dimension relative to said riser section''s height and width, said hood projecting forwardly from said riser section horizontally over generally the entire rear portion of the cooking top, the height of said hood section being small relative to the width and depth of same wherein the improvement comprises an atmospheric air exhaust system including an air inlet in the front splash wall of said riser section for admitting vapors rising from the cooking top thereto and an air exit in the rear wall Of said riser section adapted for discharge by ducting through a building wall to the atmosphere, said splash wall air inlet and said rear wall air exit being substantially in alignment, a vertical shroud wall spaced intermediate said front splash wall and said rear wall dividing said riser section into a front vertical low pressure air passage in communication with said splash wall air inlet and a rear vertical high pressure plenum chamber in communication with said rear wall air exit, first and second rotatable propeller single speed fans having rotatable propeller blades supported in side-by-side relation in said hood, said shroud wall having a pair of propeller blade orifice means arranged in side-by-side relation surrounding respectively each of said first and second axial flow propeller blades, a unitary double-lobed fan housing positioned in said plenum chamber, each lobe of said double-lobed fan housing having offset first and second apertures receiving said first and second orifice means therethrough forming a first U-shaped casing enclosing said first propeller blade and a second scroll-shaped casing enclosing said second propeller blade, each said casing having downwardly opening air outlets, said double-lobed housing having a common air exit positioned in aligned flush communication with said rear wall air exit, said U-shaped casing and said scroll-shaped casing operative to convert said first and second axial flow propeller fans into mixed radial and axial flow fans, said U-shaped casing enclosing said first propeller blade in a manner whereby said first blade has unrestricted exposure to its casing outlet and said scroll-shaped casing enclosing said second blade, whereby said second blade has about two-thirds exposure to its casing outlet to enable said second fan to develop sufficient static pressure such that it may be energized singly to provide normal air exhaust flow through a building wall ducting to the atmosphere while said first fan may be energized to run in unison with said second fan to provide for high air exhaust flow to the atmosphere.
 4. The claimed subject matter of claim 3 wherein said rear wall air exit has a restricted rectangular shape of a size to mate with a conventional rectangular wall duct having the dimensions of approximately 3 inches in height and 10 inches in width.
 5. The claimed subject matter of claim 3 wherein said shroud wall orifice means are in the form of cylindrical duct orifices. 